Communication management apparatus, communication apparatus, and communication method

ABSTRACT

A communication management apparatus includes a token-circulation-order-information storing unit that stores token circulation order, a token-frame processing unit that determines whether transmission right acquiring apparatus information of the token frame indicates the own apparatus and transmits a token frame in which first transmission right acquisition determination information indicating a sequence number of a communication apparatus that can acquire a transmission right in the token circulation order, second transmission right acquisition determination information indicating the number of frames that can be transmitted during one token frame circulation, and transmission right acquiring apparatus information indicating the next transmission destination of the token frame after the own apparatus acquired from the token circulation order are set, and a data-frame-communication processing unit that transmits and receives the data frame, thereby to reduce time required until a communication node that acquired the token frame transmits data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.13/142,239 filed Jun. 24, 2011, which is a 371 of PCT Application No.PCT/JP2008/073639 filed Dec. 25, 2008; the above-noted applications areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a communication management apparatus, acommunication apparatus, and a communication method for performingcommunication using a token frame among communication nodes connected byan Ethernet.

BACKGROUND ART

In the past, there is known a method of controlling, in a local areanetwork in which a plurality of communication nodes are connected,transmission of data from the communication nodes using a token passingsystem for circulating a token frame that indicates a transmission right(see, for example, Patent Document 1). In the technology described inPatent Document 1, an allowed transmission data amount, which is anamount of data that can be transmitted while the token frame iscirculated in a network once, is set in the token frame and, when anamount of data that should be transmitted by a communication node thathas acquired the token frame is equal to or smaller than the allowedtransmission data amount set in the token frame, after the data istransmitted, a token frame in which an amount of data obtained bysubtracting the transmitted amount of data from the allowed transmissiondata amount is set as a new allowed transmission data amount istransmitted to the next communication node. When the amount of data thatshould be transmitted is larger than the allowed transmission dataamount, the communication node transmits a token frame in which theallowed transmission data amount is set to 0 to the next node andmemorizes that the allowed transmission data amount is set to 0. Even ifcommunication nodes having turns of token circulation order after theturn of this communication node acquire the token frame, thecommunication nodes cannot transmit data because the allowedtransmission data amount is “0”. Thereafter, when a management stationin the network receives the token frame in which the allowedtransmission data amount is set to 0, the management station transmits atoken frame in which the allowed transmission data amount is set to“−1”. When the communication node that sets the allowed transmissiondata amount of the token frame received last time to “0” receives thetoken frame in which the allowed transmission data amount is set to“−1”, the communication node obtains a transmission right of data, setsthe allowed transmission data amount to an initial value, and transmitsdata that could not be transmitted last time.

Patent Document 1: Japanese Patent No. 3487324

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

However, in the communication method described in Patent Document 1,when a communication node that acquires a token frame transmits data,the communication node calculates an amount of data that should betransmitted and compare the calculated amount of data that should betransmitted and an allowed transmission data amount included in thetoken frame. Therefore, there is a problem in that time is requireduntil the communication node transmits the data after acquiring thetoken frame.

In the method described in Patent Document 1, for example, acommunication node that has received a token frame for the nth time butcould not transmit data because an amount of data that should betransmitted is larger than the allowed transmission data amount cansurely transmit data when the communication node acquires the tokenframe next for the (n+1)th time. However, after the reception of the nthtoken frame, when data that should be transmitted to this communicationnode is added anew while the token frame is circulated once, when thecommunication node acquires the token frame for the (n+1)th time, anamount of data obtained by adding the amount of the added data to anamount of data that should have been transmitted when the token framehas received for the nth time is transmitted. Therefore, there is also aproblem in that, for example, when a communication node located on adownstream side of the communication node stores the data that should betransmitted when the token frame is received for the nth time, deviationoccurs in time order of data that should be transmitted. As a result,the data cannot be transmitted among communication nodes on schedulesubjected to transmission entry.

The present invention has been devised in view of the above and it is anobject of the present invention to obtain, in a communication systemthat circulates a token frame in a network and performs communication, acommunication management apparatus, a communication apparatus, and acommunication method that can reduce, compared with the past, timerequired until a communication node transmits data after acquiring atoken frame. It is also an object of the present invention to obtain acommunication management apparatus, a communication apparatus, and acommunication method that can transmit, generally in order of generationof data, the data transmitted by communication nodes included in thecommunication system.

Means for Solving Problem

In order to solve the aforementioned problems, a communicationmanagement apparatus that manages transmission of data in a network inwhich one communication management apparatus and one or morecommunication apparatuses are connected by an Ethernet cable accordingto one aspect of the present invention is constructed in such a manneras to include: a token-circulation-order storing unit that stores tokencirculation order for circulating a token frame in the network; atoken-frame receiving unit that determines whether transmission rightacquiring apparatus information for acquiring a transmission right nextin the received token frame indicates the own apparatus; a token-frametransmitting unit that transmits the token frame in which firsttransmission right acquisition determination information indicating asequence number of a communication apparatus that can acquire thetransmission right in the token circulation order, second transmissionright acquisition determination information indicating a number offrames that can be transmitted while the token frame circulates oncefrom the own apparatus, and transmission right acquiring apparatusinformation that indicates a next transmission destination of the tokenframe after the own apparatus acquired from the token circulation orderare set; and a data-frame-communication processing unit that performsreception processing for data frames from other communication nodes and,when the transmission right is acquired, converts data into a data framein frame unit and transmits the data frame.

Effect of the Invention

According to the present invention, when a communication node in acommunication system transmits data, the communication managementapparatus performs first determination for determining, according tofirst transmission right acquisition determination information, whetherthe communication node is a communication node that can transmit dataand, when the communication node can transmit data, causes thecommunication node to transmit data within a range of the number offrames of second transmission right acquisition determinationinformation. As a result, communication nodes only have to transmit, inframe unit, data accumulated in transmission buffers. Therefore, thereis an effect that it is unnecessary to calculate an amount of data thatcan be transmitted and it is possible to reduce, compared with the past,time required until a communication node transmits data after acquiringthe token frame. There is also an effect that, because transmission iscarried out in transmission entry order, it is easy to perform feedbackcontrol used in a control apparatus.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an example of the configuration of acommunication system according to a first embodiment of the presentinvention.

FIG. 2-1 is a schematic block diagram of a functional configuration of amaster station.

FIG. 2-2 is a schematic block diagram of a functional configuration of aslave station.

FIG. 3 is a diagram of an example of a format of a token frame used inthe first embodiment.

FIG. 4 is a schematic diagram of an example of a storage state of datain a transmission buffer.

FIG. 5-1 is a flowchart for explaining an example of transmissionprocessing at the time of reception of the token frame in the slavestation (first).

FIG. 5-2 is a flowchart for explaining the example of the transmissionprocessing at the time of reception of the token frame in the slavestation (second).

FIG. 6 is a flowchart for explaining an example of processing at thetime of reception of the token frame in the master station.

FIG. 7-1 is a schematic diagram for explaining an example of acommunication method employing the token frame according to the firstembodiment (first).

FIG. 7-2 is a schematic diagram for explaining the example of thecommunication method employing the token frame according to the firstembodiment (second).

FIG. 7-3 is a schematic diagram for explaining the example of thecommunication method employing the token frame according to the firstembodiment (third).

FIG. 7-4 is a schematic diagram for explaining the example of thecommunication method employing the token frame according to the firstembodiment (fourth).

FIG. 7-5 is a schematic diagram for explaining the example of thecommunication method employing the token frame according to the firstembodiment (fifth).

FIG. 7-6 is a schematic diagram for explaining the example of thecommunication method employing the token frame according to the firstembodiment (sixth).

FIG. 7-7 is a schematic diagram for explaining the example of thecommunication method employing the token frame according to the firstembodiment (seventh).

FIG. 7-8 is a schematic diagram for explaining the example of thecommunication method employing the token frame according to the firstembodiment (eighth).

FIG. 7-9 is a schematic diagram for explaining the example of thecommunication method employing the token frame according to the firstembodiment (ninth).

FIG. 8 is a flowchart for explaining an example of processing at thetime of reception of a token frame by a master station.

FIG. 9-1 is a schematic diagram for explaining an example of a procedureof a communication method employing the token frame according to asecond embodiment (first).

FIG. 9-2 is a schematic diagram for explaining the example of theprocedure of the communication method employing the token frameaccording to the second embodiment (second).

FIG. 9-3 is a schematic diagram for explaining the example of theprocedure of the communication method employing the token frameaccording to the second embodiment (third).

FIG. 9-4 is a schematic diagram for explaining the example of theprocedure of the communication method employing the token frameaccording to the second embodiment (fourth).

FIG. 9-5 is a schematic diagram for explaining the example of theprocedure of the communication method employing the token frameaccording to the second embodiment (fifth).

FIG. 9-6 is a schematic diagram for explaining the example of theprocedure of the communication method employing the token frameaccording to the second embodiment (sixth).

FIG. 9-7 is a schematic diagram for explaining the example of theprocedure of the communication method employing the token frameaccording to the second embodiment (seventh).

FIG. 9-8 is a schematic diagram for explaining the example of theprocedure of the communication method employing the token frameaccording to the second embodiment (eighth).

FIG. 10 is a schematic block diagram of a functional configuration of amaster station according to a third embodiment.

FIG. 11 is a schematic diagram for explaining a relation among stationnumbers, sequence numbers, and priority levels of communication nodesincluded in a communication system.

FIG. 12-1 is a flowchart for explaining an example of a token frameprocessing procedure of the master station according to the thirdembodiment (first).

FIG. 12-2 is a flowchart for explaining the example of the token frameprocessing procedure of the master station according to the thirdembodiment (second).

FIG. 13-1 is a schematic diagram for explaining an example of tokenframe transmission processing according to the third embodiment (first).

FIG. 13-2 is a schematic diagram for explaining the example of the tokenframe transmission processing according to the third embodiment(second).

FIG. 13-3 is a schematic diagram for explaining the example of the tokenframe transmission processing according to the third embodiment (third).

FIG. 13-4 is a schematic diagram for explaining the example of the tokenframe transmission processing according to the third embodiment(fourth).

FIG. 13-5 is a schematic diagram for explaining the example of the tokenframe transmission processing according to the third embodiment (fifth).

FIG. 14 is a diagram of an example of a format of a token frame used ina fourth embodiment.

EXPLANATIONS OF LETTERS OR NUMERALS

11-1, 11-2, 51-1, 51-2 Ports

20, 60 Communication processing units

21 Logical-ring configuring unit

22 Token-circulation-order-information storing unit

23, 62 Token-frame processing units

24, 63 Data-frame-communication processing units

25, 64 Transmission buffers

26, 65 Reception buffers

27 Changed-destination-information-during-priority-processing storingunit

28 Number-of-transmittable-frames “0” setting-station storing unit

30 Arithmetic processing unit

61 Token-circulation-destination-information storing unit

101 Switching hub

200 Token frame

201 Destination address (DA)

202 Transmission source address (SA)

203 Ethernet type

204 Data

205 Frame type information

206 Transmission right acquiring apparatus information

207 Token repeat counter

208 Transmission permitted station value

208 a Start transmission permitted station value

208 b End transmission permitted station value

209 Number of transmittable frames

210 Number-of-transmittable-frames “0” setting station

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Preferred embodiments of a communication management apparatus, acommunication apparatus, and a communication method according to thepresent invention are explained in detail below with reference to theaccompanying drawings. The present invention is not limited by theseembodiments.

First Embodiment

FIG. 1 is a schematic diagram of an example of the configuration of acommunication system according to a first embodiment of the presentinvention. This communication system includes a network of the samesegment in which a plurality of communication nodes A to C are connectedin a star shape by an Ethernet, the same applies in the followingexplanation) via a switching hub 101. The communication nodes A to Crespectively have ports. The ports of the communication nodes areconnected via a cable capable of performing full duplex communicationsuch as a twist pair cable or an optical fiber. In this example, as thecommunication nodes, one master station A as a management station thatmanages transmission and reception of data (frames) in the network ofthe same segment and two slave stations B and C that performtransmission of data (frames) based on order of transmission by themaster station A are provided. In this example, the communication systemincludes the communication nodes A to C connected in the star shape.However, the communication system can include communication nodesconnected in another connection form such as a line shape or a ringshape. Connection forms of the star shape and the line shape can bemixed.

In the first embodiment, in the communication system in which thecommunication nodes A to C are connected by the Ethernet, thecommunication nodes A to C do not freely perform transmission of data.Instead, a frame for obtaining a data transmission right called token (atoken frame) is transmitted in order to the communication nodes A to Cin the communication system to enable a communication node, whichacquires the token frame, to perform transmission of data to the othercommunication nodes. Transmission order of the token frame is asindicated by (1) below.Master station A→slave station B→slave station C→master station A  (1)

In this way, the communication system does not have a ring configurationin a physical network configuration. However, the data transmissionright (the token frame) is circulated in order among the communicationnodes A to C in the communication system to return the transmissionright to the ring management station X, whereby the transmission rightis repeated in a logical ring configuration.

FIG. 2-1 is a schematic block diagram of a functional configuration of amaster station. The master station includes two ports 11-1 and 11-2 forconnecting an Ethernet cable between the master station and acommunication node (a slave station) adjacent to the master station orbetween the master station and the switching hub 101, a communicationprocessing unit 20 that performs, for example, transmission andreception processing for a frame via the ports 11-1 and 11-2 andprocessing for establishing transmission order of a token frame, and anarithmetic processing unit 30 that performs arithmetic processing usingdata from a slave station.

The ports 11-1 and 11-2 include two ports, i.e., a first port 11-1 and asecond port 11-2. At least one of these two ports 11-1 and 11-2 only hasto be connected to a port of the slave station adjacent to the masterstation (or connected to a port of a slave station via the switching hub101).

The communication processing unit 20 includes a logical-ring configuringunit 21, a token-circulation-order-information storing unit 22, atoken-frame processing unit 23, a data-frame-communication processingunit 24, a transmission buffer 25, and a reception buffer 26.

The logical-ring configuring unit 21 detects, during power-on of themaster station or at every predetermined time, a communication node (aslave station) present in a network of the same segment as the masterstation and performs logical ring configuration processing fordetermining token circulation order information that is order forfeeding (passing) a token frame, which is a data transmission right,from a connection relation between the master station and thecommunication nodes. The logical-ring configuring unit 21 notifies othercommunication nodes (slave stations) present in the network of the samesegment of token circulation destination information including acommunication node that obtains the transmission right after thecommunication node. This token circulation destination information canbe token circulation order information.

The token-circulation-order-information storing unit 22 stores the tokencirculation order information determined by the logical-ring configuringunit 21.

When the logical ring configuration processing by the logical-ringconfiguring unit 21 ends, the token-frame processing unit 23 generates atoken frame, performs transmission processing for the token frame basedon the token circulation order information, and, when the token frame isreceived, determines whether the transmission right is acquired. FIG. 3is a diagram of an example of a format of the token frame used in thefirst embodiment. A token frame 200 is an Ethernet frame and has adestination MAC (Media Access Control) address (DA) 201, a transmissionsource MAC address (SA) 202, an Ethernet type (type) 203, a data 204 inwhich data of higher layers is stored, and an FCS (Frame Check Sequence)211 that stores a result of check concerning whether there is an errorin information stored in the DA 201 to the data 204 of the own frame.

In the first embodiment, frame type information 205, transmission rightacquiring apparatus information 206, a token repeat counter 207, atransmission permitted station value 208, a number of transmittableframes 209, and a number-of-transmittable-frames “0” setting station 210are stored in a part of the data 204.

Information indicating a type of an Ethernet frame is stored in theframe type information 205. In this case, information indicating thatthe frame is a token frame is stored. A MAC address of a communicationnode having a transmission right is stored in the transmission rightacquiring apparatus information 206. This transmission right in thetransmission right acquiring apparatus information 206 means a state inwhich it is possible to acquire the token frame and acquire thetransmission right. The transmission right is not a right that makes itpossible to actually perform transmission of data.

The token repeat counter 207 is a counter that counts a sequence numbercounted from the master station in a logical ring of a communicationnode that receives the token frame 200. The sequence number correspondsto a token circulation number in claims. The token repeat counter 207indicates, with the master station set as a 0th communication node, inwhich position counted from the master station the communication nodeindicated by the transmission right acquiring apparatus information 206is located on the logical ring. For example, when the communicationnodes have the token circulation order of (1) above in the configurationshown in FIG. 1, the slave station B is a first communication node onthe logical ring and the slave station C is a second communication nodeon the logical ring.

The transmission permitted station value 208 indicates a sequence numberof a communication node that can transmit a data frame first among thecommunication nodes on the logical ring. This sequence number is thesame as a value counted by the token repeat counter 207. The tokenrepeat counter 207 and the transmission permitted station value 208configure first transmission right acquisition determination informationof claims.

The number of transmittable frames 209 represents a number of framesthat can be transmitted while the token frame 200 is transmitted fromthe master station and circulates once. Thenumber-of-transmittable-frames “0” setting station 210 indicates asequence number of a communication node in which the number oftransmittable frames 209 is set to “0”. This sequence number is the sameas a value counted by the token repeat counter 207.

Reception processing and transmission processing performed when such atoken frame 200 is used are explained below. When the token-frameprocessing unit 23 receives the token frame 200 transmitted from anothercommunication node (slave station), the token-frame processing unit 23compares the transmission right acquiring apparatus information 206 inthe data 204 of the token frame 200 and a MAC address of the ownstation. When the transmission right acquiring apparatus information 206and the MAC address of the own station coincide with each other, thetoken-frame processing unit 23 determines that the communication node isin a state in which the communication node can transmit data. When theown station can transmit a data frame according to the informationstored in the data 204, the token-frame processing unit 23 gives aninstruction to the data-frame-communication processing unit 24 toperform transmission processing for a data frame. However, when the ownstation cannot transmit a data frame, the token-frame processing unit 23determines that the transmission right could not be acquired. When thetransmission right acquiring apparatus information 206 and the MACaddress of the own station do not coincide with each other, thetoken-frame processing unit 23 determines that the transmission right isnot obtained yet. In any case, the received token frame 200 is repeatedto another port that is not a port where the token frame 200 isreceived. The reception processing from the token frame 200 by thetoken-frame processing unit 23 corresponds to token-frame receivingapparatus of claims.

When the token-frame processing unit 23 receives the token frame 200 inwhich the transmission right acquiring apparatus information 206indicates the own station, the token-frame processing unit 23 sets thetransmission right acquiring apparatus information 206, the token repeatcounter 207, the transmission permitted station value 208, and thenumber of transmittable frames 209 and transmits the token frame 200 inbroadcast. The token-frame processing unit 23 sets the token repeatcounter 207 to “1”, sets the number of transmittable frames 209 to apredetermined value decided in advance, and sets, as the transmissionright acquiring apparatus information 206, (a MAC address of) a slavestation that obtains, based on the token circulation order information,the transmission right after the own station.

When the number of transmittable frames 209 at the time of reception ofa token frame is not “0”, the token-frame processing unit 23 sets “0”,which is sequence order of the own station in the logical ring, in thetransmission permitted station value 208. When the number oftransmittable frames 209 at the time of reception of a token frame is“0”, the token-frame processing unit 23 sets a value stored in thenumber-of-transmittable-frames “0” setting station 210 of the tokenframe 200 at the reception point in the transmission permitted stationvalue 208. The transmission processing for the token frame 200 by thetoken-frame processing unit 23 corresponds to a token-frame transmittingunit in claims.

The data-frame-communication processing unit 24 performs transmissionand reception processing for a data frame. Specifically, thedata-frame-communication processing unit 24 also has a function of, whenthe transmission right is acquired, converting data accumulated in thetransmission buffer 25 into a data frame and transmitting the data frameto a slave station, subjecting a data frame accumulated in the receptionbuffer 26 from the slave station to reception processing, andtransferring (repeating) a data frame addressed to another slave stationby the slave station.

The transmission buffer 25 temporarily stores data frames calculated bythe arithmetic processing unit 30 and transmitted to the othercommunication nodes by the data-frame-communication processing unit 24.The data frames are transmitted in order of the storage in thetransmission buffer 25.

The reception buffer 26 temporarily stores data frames from the othercommunication nodes received in the ports 11-1 and 11-2. The receptionbuffer 26 stores data frames received in frame unit and can store only apredetermined number of frames.

The arithmetic processing unit 30 performs a predetermined arithmeticoperation for generating information and the like at a predeterminedcycle for controlling the other communication nodes using data from theother communication nodes subjected to reception processing by thedata-frame-communication processing unit 24. An arithmetic processingresult is transmitted to the other communication nodes via thedata-frame-communication processing unit 24.

FIG. 2-2 is a schematic block diagram of a functional configuration of aslave station. The slave station includes two ports 51-1 and 51-2 forconnecting an Ethernet cable between the slave station and acommunication node (a ring management station or a slave station)adjacent to the slave station or between the slave station and theswitching hub 101 and a communication processing unit 60 that performstransmission and reception processing for a frame via the ports 51-1 and51-2.

As in the master station, the ports 51-1 and 51-2 include two ports,i.e., a first port 51-1 and a second port 51-2. At least one of thesetwo ports 51-1 and 51-2 only has to be connected to a communicationnode.

The communication processing unit 60 includes atoken-circulation-destination-information storing unit 61, a token-frameprocessing unit 62, a data-frame-communication processing unit 63, atransmission buffer 64, and a reception buffer 65.

The token-circulation-destination-information storing unit 61 storestoken circulation destination information notified from the masterstation. It is assumed that only a MAC address of a communication nodethat obtains the transmission right after the own slave station isstored as token circulation destination information.

When the token-frame processing unit 62 receives the token frame 200transmitted from another communication node, the token-frame processingunit 62 compares the transmission right acquiring apparatus information206 in the data 204 of the token frame 200 and a MAC address of the ownstation (slave station). When the transmission right acquiring apparatusinformation 206 and the MAC address coincide with each other, thetoken-frame processing unit 62 determines that the slave station is in astate in which the slave station can perform transmission and compares avalue of the token repeat counter 207 of the received token frame 200and a value of the transmission permitted station value 208. When thetoken repeat counter 207≧the transmission permitted station value 208,the token-frame processing unit 62 determines that the transmissionright for a data frame has been obtained within the range of the numberof transmittable frames 209. However, when the number of transmittableframes 209 is “0”, the slave station cannot transmit a data frame. Onthe other hand, when the token repeat counter 207<the transmissionpermitted station value 208, the token-frame processing unit 62determines that the transmission right for a data frame has not beenobtained. When the transmission right acquiring apparatus information206 and the MAC address of the own station do not coincide with eachother, the token-frame processing unit 62 determines that the slavestation is not in the state in which the slave station can performtransmission. The processing for receiving a token frame in thetoken-frame processing unit 62 corresponds to a token-frame receivingunit of claims.

The token-frame processing unit 62 sets, with respect to the receivedtoken frame 200, the transmission right acquiring apparatus information206, the token repeat counter 207, and the number of transmittableframes 209 and sets the number-of-transmittable-frames “0” settingstation 210 if necessary and transmits the token frame 200 in broadcast.

Specifically, the token-frame processing unit 62 sets the token repeatcounter 207 obtained by incrementing the token repeat counter 207 storedin the token frame 200 by one and sets, as the transmission rightacquiring apparatus information 206, the MAC address of thecommunication node set in the token circulation destination information.

When data frames are transmitted, the token-frame processing unit 62sets, in the number of transmittable frames 209, a value obtained bysubtracting the number of transmitted data frames from the number oftransmittable frames 209 at the time of token frame reception. When thenew number of transmittable frames 209 is “0”, the token-frameprocessing unit 62 sets, as the number-of-transmittable-frames “0”setting station 210, a value of the token repeat counter 207 at the timeof the token frame 200 reception. The processing for transmitting atoken frame in the token-frame processing unit 62 corresponds to atoken-frame transmitting unit of claims.

When the data transmission right is obtained, thedata-frame-communication processing unit 63 performs transmission andreception processing for a data frame within a range specified by thenumber of transmittable frames 209. For example, in an FA network, thedata-frame-communication processing unit 63 receives data transmittedfrom the master station and set in the slave station and transmits dataused in arithmetic processing to the master station. Thedata-frame-communication processing unit 63 also has a function ofreceiving a data frame transmitted from the slave station andtransferring (repeating) a data frame addressed to another slave stationby the slave station.

The data-frame-communication processing unit 63 performs processing fordistinguishing data (a frame) that could not be transmitted because thenumber of transmittable frames is “0” in the own station and data (aframe) that could not be transmitted because, although the token frame200 is acquired, the number of transmittable frames is “0” from dataaccumulated in the transmission buffer 64 after the token frame 200 isreleased. This processing is explained later.

The transmission buffer 64 temporarily stores data entered in a datalink layer from a not-shown application layer of the slave station untilthe data is transmitted by the data-frame-communication processing unit63. The reception buffer 65 temporarily stores data received in theports 51-1 and 51-2 until the data is subjected to reception processingby the data-frame-communication processing unit 63.

FIG. 4 is a schematic diagram of an example of a storage state of datain a transmission buffer. As shown in the figure, in the transmissionbuffer, data that should be transmitted to other communication nodes(mainly, the master station) is accumulated in time series astransmission waiting frames. Information for distinguishing data (aframe) that, although the transmission right was acquired, was not ableto be transmitted is stored with respect to the respective transmissionwaiting frames. In this embodiment, the transmission buffer hasun-transmitted frame information at the time of number of transmittableframes “0” setting for identifying, when the number of transmittableframes 209 in the own station is “0”, data (a frame) that could not betransmitted and non-transmitted frame information at the time of lasttoken reception for identifying data (a frame) that could not betransmitted because the transmission right was not obtained (i.e., thenumber of transmittable frames 209 is “0”). The un-transmitted frameinformation at the time of number of transmittable frames “0” settingand the un-transmitted frame information at the time of last tokenreception correspond to last-time un-transmitted data identificationinformation of claims.

Processing at the time of token frame reception is explained below inorder of the slave station and the master station. FIGS. 5-1 and 5-2 areflowcharts for explaining an example of transmission processing at thetime of token frame reception in the slave station.

First, when the token-frame processing unit 62 receives the token frame200 (step S11), the token-frame processing unit 62 determines whetherthe transmission right acquiring apparatus information 206 of the tokenframe 200 indicates the own station (step S12). When the transmissionright acquiring apparatus information 206 does not indicate the ownstation (No at step S12), the token-frame processing unit 62 transfersthe token frame (step S13) and the processing ends.

When the transmission right acquiring apparatus information 206indicates the own station (Yes at step S12), the token-frame processingunit 62 determines whether a value of the token repeat counter 207 ofthe received token frame 200 is equal to or larger than the transmissionpermitted station value 208 (step S14). When the value of the tokenrepeat counter 207 is equal to or larger than the transmission permittedstation value 208 (Yes at step S14), the token-frame processing unit 62further determines whether the number of transmittable frames 209 of thetoken frame 200 is larger than “0” (step S15).

When the number of transmittable frames 209 is larger than “0” (Yes atstep S15), the token-frame processing unit 62 recognizes that the slavestation has acquired the transmission right. The token-frame processingunit 62 determines whether the number of frames stored in thetransmission buffer 64 at a point when the token frame 200 is received(hereinafter referred to as number of transmission waiting frames) isequal to or smaller than the number of transmittable frames (step S16).When the number of transmission waiting frames is equal to or smallerthan the number of transmittable frames (Yes at step S16), thetoken-frame processing unit 62 further determines whether anun-transmitted frame at the time of number of transmittable frames “0”setting or an un-transmitted frame at the time of last token receptionis present in the transmission buffer 64 (step S17).

When an un-transmitted frame at the time of number of transmittableframes “0” setting or an un-transmitted frame at the time of last tokenreception is not present in the transmission buffer 64 (No at step S17),the token-frame processing unit 62 performs transmission processing forall frames stored in the transmission buffer (step S21). When anun-transmitted frame at the time of number of transmittable frames “0”setting or an un-transmitted frame at the time of last token receptionis present in the transmission buffer 64 (Yes at step S17), thedata-frame-communication processing unit 63 transmits only theun-transmitted frame at the time of number of transmittable frames “0”setting or the un-transmitted frame at the time of last token reception(step S22).

Thereafter, the token-frame processing unit 62 sets (the number oftransmittable frames 209 at the time of token frame 200 reception)−(thenumber of frames transmitted at step S21 or S22) in the number oftransmittable frames 209 of the token frame 200 (step S23).Subsequently, the token-frame processing unit 62 determines whether anew number of transmittable frames 209 set in the token frame 200 is “0”(step S24).

When the set number of transmittable frames 209 is not “0” (No at stepS24), the token-frame processing unit 62 increments the value of thetoken repeat counter 207 of the token frame 200 by one (step S41) andsets, in the transmission right acquiring apparatus information 206,token circulation destination information stored in thetoken-circulation-destination-information storing unit 61 (step S42).The token-frame processing unit 62 transmits the token frame 200 inwhich the items in the data 204 are set as explained above (step S43)and ends the processing.

When the number of transmittable frames 209 set at step S23 is 0 (Yes atstep S24), the token-frame processing unit 62 sets, in thenumber-of-transmittable-frames “0” setting station 210 of the tokenframe 200, the value of the token repeat counter 207 at the time of thetoken frame 200 reception (step S34). Thereafter, the processing at stepS41 and subsequent steps is performed.

On the other hand, when the number of transmission waiting frames islarger than the number of transmittable frames 209 at step S16 (No atstep S16), the data-frame-communication processing unit 63 transmits theframes of the number of transmittable frames 209 stored in thetransmission buffer 64 in order from a frame stored earliest (step S31).At this point, the token-frame processing unit 62 sets, innon-transmitted frames among the frames stored in the transmissionbuffer 64, a flag for identifying un-transmitted frame at the time ofnumber of transmittable frames “0” setting (step S32). The token-frameprocessing unit 62 sets “0”, i.e., (the number of transmittable frames209 at the time of the token frame 200 reception)−(the number of framestransmitted at step S31) in the number of transmittable frames 209 ofthe token frame 200 (step S33) and sets the number of transmittableframes 209 of the token frame 200 to “0”. Thereafter, the processing atstep S34 and subsequent steps is performed.

When the number of transmittable frames 209 is “0” at step S15 (No atstep S15), the token-frame processing unit 62 recognizes that thetransmission right has not been obtained. The data-frame-communicationprocessing unit 63 sets, in the frames stored in the transmission buffer64 at the point when the token frame 200 is received, a flag foridentifying an un-transmitted frame at the time of last token reception(step S25). Thereafter, the processing at step S41 and subsequent stepsis performed.

When the value of the token repeat counter 207 is smaller than thetransmission permitted station value 208 at step S14 (No at step S14),the token-frame processing unit 62 of the slave station recognizes thatthe transmission right has not been obtained. The processing at step S41and subsequent steps is performed. As explained above, the processing ofa token frame by the slave station is performed.

The processing at steps S34 to S42 are setting processing forinformation in the data 204 of the token frame 200. Therefore, order ofsetting is not limited to the order explained above.

FIG. 6 is a flowchart for explaining an example of processing at thetime of token frame reception in the master station. First, when thetoken-frame processing unit 23 receives the token frame 200 (step S71),the token-frame processing unit 23 determines whether the transmissionright acquiring apparatus information 206 of the token frame 200indicates the own station (step S72). When the transmission rightacquiring apparatus information 206 does not indicate the own station(No at step S72), the token-frame processing unit 23 transfers the tokenframe 200 (step S73) and the processing ends.

When the transmission right acquiring apparatus information 206indicates the own station (Yes at step S72), the token-frame processingunit 23 checks whether the number of transmittable frames 209 of thereceived token frame 200 is “0” (step S74). When the number oftransmittable frames is not “0” (No at step S74), the token-frameprocessing unit 23 of the master station recognizes that a communicationnode (a slave station) that cannot transmit a data frame is not present.In the next circulation of the token frame 200, the token-frameprocessing unit 23 performs setting processing for the token frame 200to give the transmission right to the communication nodes in orderaccording to the token circulation order information. Specifically,after the data-frame-communication processing unit 24 transmits a framein the transmission buffer 25 within a range of the number oftransmittable frames set in advance (step S75), the token-frameprocessing unit 23 sets, in the number of transmittable frames 209 ofthe token frame 200, a value obtained by subtracting the number offrames transmitted at step S75 from the number of transmittable framesset in advance (step S76), sets the value of the token repeat counter207 to “1” (step S77), and sets the transmission permitted station value208 to “0” (step S78).

On the other hand, when the number of transmittable frames 209 is “0” atstep S74 (Yes at step S74), the token-frame processing unit 23 of themaster station recognizes that a communication node (a slave station)that was not able to transmit a data frame while this token frame 200circulated once is present. The token-frame processing unit 23 performssetting processing for the token frame 200 such that, in the nextcirculation of the token frame 200, data frames can be transmitted inorder from the communication node (the slave station) that was not ableto transmit a data frame. Specifically, the token-frame processing unit23 sets the number of transmittable frames 209 of the token frame 200 toa value set in advance (step S81), sets the value of the token repeatcounter 207 to “1” (step S82), and sets a value of thenumber-of-transmittable-frames “0” setting station 210 at the time oftoken frame 200 reception in the transmission permitted station value208 (step S83).

Thereafter or after step S78, the token-frame processing unit 23 sets,in the transmission right acquiring apparatus information 206 of thetoken frame 200, a slave station, which obtains the transmission rightafter the own station, acquired from the token circulation orderinformation in the token-circulation-order-information storing unit 22(step S91). The token-frame processing unit 23 transmits a token frameset as explained above (step S92) and the processing ends.

The processing at steps S76 to S78 and S91 is setting processing forinformation in the data 204 of the token frame 200. Therefore, order ofthe processing does not have to be the order explained above. The sameapplies to the processing at steps S81 to S83 and S91.

A communication method employing a token frame in such a communicationsystem is explained below with reference to a specific example. FIGS.7-1 to 7-9 are schematic diagrams for explaining an example of acommunication method employing a token according to the firstembodiment. It is assumed that the master station A has alreadyperformed configuration processing for a logical ring, determined thetoken circulation order indicated by (1) above, and acquired thetransmission right first. In FIG. 7-1, a state of the communicationsystem at a point when the master station A has acquired thetransmission right is shown. In this state, in the master station A, forexample, it is entered from an application layer to a data link layer totransmit frames F1 to F5 to the other slave stations B and C. Theseframes F1 to F5 are stored in the transmission buffer 25 in order astransmission waiting frames. Further, in the slave station B, it isentered from an application layer to a data link layer to transmitframes F6 and F7 to the other communication nodes. These frames F6 andF7 are stored in the transmission buffer in order as transmissionwaiting frames. In the slave station C, it is entered from anapplication layer to a data link layer to transmit a frame F8 to theother communication nodes. This frame F8 is stored in the transmissionbuffer as a transmission waiting frame.

Thereafter, as shown in FIG. 7-2, the token-frame processing unit 23 ofthe master station A sets the number of transmittable frames in thetoken frame 200 to a value set in the communication system in advance(assumed to be “6”). The data-frame-communication processing unit 24 ofthe master station A transmits the transmission waiting frames, whichare stored in the transmission buffer 25, in order of storage in thetransmission buffer 25 (in order from a frame stored earliest) withinthe range of the number of transmittable frames set in the token frame200. The number of transmission waiting frames F1 to F5 stored in thetransmission buffer 25 is “5” and the number of transmittable frames setin the token frame 200 is “6”. Therefore, the data-frame-communicationprocessing unit 24 transmits all the frames F1 to F5. Consequently, thenumber of frames that can be transmitted while this token frame 200circulates once is “1 (6−5)”. Thereafter, the token-frame processingunit 23 of the master station A transmits the token frame 200 in whichthe number of transmittable frames is set to “1”, the token repeatcounter is set to “1”, the transmission permitted station value is setto “0” representing the own station, and the slave station B is set inthe transmission right acquiring apparatus information. The transmittedframes F1 to F5 are erased from the transmission buffer 25.

Subsequently, when the slave station B receives the token frame 200 inwhich the transmission right acquiring apparatus information is set tothe own station, the token-frame processing unit 23 of the slave stationB compares the value of the token repeat counter in the token frame 200and the transmission permitted station value and determines whether theown station can transmit data. Because the token repeat counter “1”>thetransmission permitted station value “0”, the token-frame processingunit 23 recognizes that the slave station B has obtained a right fortransmitting data in the range of the number of transmittable frames inthe token frame 200.

Thereafter, as shown in FIG. 7-3, the data-frame-communicationprocessing unit 63 of the slave station B transmits the transmissionwaiting frames, which are stored in the transmission buffer 64, in orderof storage in the transmission buffer 64 (in order from a frame storedearliest) within the range of the number of transmittable frames set inthe token frame 200. The number of transmission waiting frames F6 to F7stored in the transmission buffer 64 is “2” and the number oftransmittable frames set in the token frame 200 is “1”. Therefore, thedata-frame-communication processing unit 63 transmits only the frame F6.Consequently, the number of frames that can be transmitted until thistoken frame 200 returns to the master station A is “0 (1−1)”.Thereafter, the token-frame processing unit 62 of the slave station Btransmits the token frame 200 in which the number of transmittableframes is set to “0”, the number-of-transmittable-stations “0” settingstation is set to “1”, which is a sequence number on the logical ring ofthe own station, the token repeat counter 207 is incremented by one to“2”, and the slave station C stored in thetoken-circulation-destination-information storing unit 61 is set as thetransmission right acquiring apparatus information. A value set in thenumber-of-transmittable-frames “0” setting station is a value set in thetoken repeat counter when the slave station B receives the token frame200 and is a sequence number counted from the master station A on thelogical ring.

The data-frame-communication processing unit 63 of the slave station Bstores, as an un-transmitted frame at the time of the number oftransmittable frames “0”, the frame F7 in the transmission buffer 64that could not be transmitted at the time of acquisition of thetransmission right of this time. This can be performed by, for example,setting, in the transmission buffer 64, an item called un-transmittedframe at the time of the number of transmittable frames “0” setting andsetting a flag in this item. To facilitate understanding of explanation,the figure is drawn such that an item “un-transmitted frame at the timeof the number of transmittable frames “0” setting” is provided and theun-transmitted frame F7 in the transmission buffer 64 is stored in thisitem as well. The transmitted frame F6 is erased.

Subsequently, the token-frame processing unit 62 of the slave station Creceives the token frame 200 in which the transmission right acquiringapparatus information is set to the own station, compares the value ofthe token repeat counter in the token frame 200 and the transmissionpermitted station value, and determines whether the own station cantransmit data. Because the token repeat counter “2”>the transmissionpermitted station value “0”, the token-frame processing unit 62recognizes that the slave station C has obtained a right fortransmitting data in the range of the number of transmittable frames inthe token frame 200.

Thereafter, as shown in FIG. 7-4, the slave station C attempts totransmit the transmission waiting frames stored in the transmissionbuffer 64 in order of storage in the transmission buffer 64 (in orderfrom a frame stored earliest) within the range of the number oftransmittable frames set in the token frame 200. Because the number oftransmittable frames set in the token frame 200 is “0”, the slavestation C cannot transmit the frame F8 in the transmission buffer 64.Therefore, the token-frame processing unit 62 of the slave station Cincrements the token repeat counter by one to “3”. Subsequently, thetoken-frame processing unit 62 transmits the token frame 200 in whichthe master station A stored in thetoken-circulation-destination-information storing unit 61 is set as thetransmission right acquiring apparatus information. In this token frame200, the number of transmittable frames remains “0” and thenumber-of-transmittable-frames “0” setting station remains “1”.

At this point, the data-frame-communication processing unit 63 of theslave station C stores, as an un-transmitted frame at the time of thelast token reception, the frame F8 in the transmission buffer 64 thatcould not be transmitted at the time of acquisition of the transmissionright of this time. This can be performed by, for example, setting anitem called un-transmitted frame at the time of the last token receptionand setting a flag in this item. To facilitate understanding of theexplanation, the figure is drawn such that an item “un-transmitted frameat the time of the last token reception” is provided and theun-transmitted frame F8 in the transmission buffer 64 is stored in thisitem as well.

Thereafter, as shown in FIG. 7-5, it is assumed that, in a stateimmediately after the slave station C transmits the token frame 200 inwhich the transmission right acquiring apparatus information is set tothe master station A, the communication nodes acquire new data thatshould be transmitted. Specifically, in the master station A, forexample, it is entered from the application layer to the data link layerto transmit frames F9 and F10 to the other slave stations. These framesF9 and F10 are stored in the transmission buffer 25 in order astransmission waiting frames. In the slave station B, it is entered fromthe application layer to the data link layer to transmit frames F11 toF14 to the other communication nodes. These frames F11 to F14 are storedin the transmission buffer 64 in order as transmission waiting frames.In the slave station C, it is entered from the application layer to thedata link layer to transmit a frame F15 to the other communicationnodes. This frame F15 is stored in the transmission buffer 64 as atransmission waiting frame.

Subsequently, as shown in FIG. 7-6, when the token-frame processing unit23 of the master station A receives the token frame 200 in which thetransmission right acquiring apparatus information is set to the ownstation, the token-frame processing unit 23 checks the number oftransmittable frames in the token frame 200. The token-frame processingunit 23 determines that, because the number of transmittable frames is“0”, a communication node that, although the transmission right wasobtained, was not able to transmit a frame, i.e., was not able to obtainthe transmission right is present among the communication nodes in thelogical ring. As a result, the data-frame-communication processing unit24 of the master station A does not perform transmission processing forthe frames F9 and F10 stored in the transmission buffer 25. Thedata-frame-communication processing unit 24 transmits the token frame200 in which a value “6” set in advance is set to the number oftransmittable frames of the received token frame 200, “1”, which is avalue of the number-of-transmittable-frames “0” setting station of thereceived token frame 200, is set to the transmission permitted stationvalue, the token repeat counter 207 is reset to “1”, and the slavestation B is set as the transmission right acquiring apparatusinformation 206.

Subsequently, when the slave station B receives the token frame 200 inwhich the transmission right acquiring apparatus information 206 is setto the own station, the data-frame-communication processing unit 24compares the value of the token repeat counter 207 in the token frame200 and the transmission permitted station value 208 and determineswhether the own station can transmit data. Because the value “1” of thetoken repeat counter 207=the transmission permitted station value 208“1”, i.e., the token repeat counter the transmission permitted stationvalue, the data-frame-communication processing unit 24 recognizes thatthe slave station B has obtained a right for transmitting data in therange of the number of transmittable frames in the token frame 200.

Thereafter, as shown in FIG. 7-7, the data-frame-communicationprocessing unit 24 of the slave station B identifies presence of“un-transmitted frames at the time of number of transmittable frames “0”setting” among the un-transmitted frames stored in the transmissionbuffer 64. The data-frame-communication processing unit 24 transmits the“un-transmitted frames at the time of number of transmittable frames “0”setting” in order of storage in the transmission buffer 64 (in orderfrom a frame stored earliest) within the range of the number oftransmittable frames set in the token frame 200. Because, although theframes F7 and F11 to F14 are stored in the transmission buffer 64, the“un-transmitted frame at the time of number of transmittable frames “0”setting” is the frame F7, the data-frame-communication processing unit24 transmits only the frame F7. Consequently, the number of frames thatcan be transmitted until this token frame 200 returns to the masterstation A is “5 (=6−1)”. Thereafter, the token-frame processing unit 62of the slave station B transmits the token frame 200 in which “5” is setin the number of transmittable frames, the token repeat counter isincremented by one to “2”, and the slave station C is set in thetransmission right acquiring apparatus information. The slave station Bstores, as the “un-transmitted frames at the time of last tokenreception”, the frames F11 to F14 that were not able to be transmittedat the time of acquisition of the transmission right of this time. Thenumber-of-transmittable-frames “0” setting station in the token frame200 is not changed and is kept at the value set in the received tokenframe 200. The transmitted frame F7 is erased from the transmissionbuffer 64.

Subsequently, the slave station C receives the token frame 200 in whichthe transmission right acquiring apparatus information is set to the ownstation, compares the value of the token repeat counter in the tokenframe 200 and the transmission permitted station value, and determineswhether the own station can transmit data. Because the token repeatcounter “2”>the transmission permitted station value “1”, the slavestation C recognizes that the slave station C has obtained a right fortransmitting data in the range of the number of transmittable frames inthe token frame 200.

Thereafter, as shown in FIG. 7-8, the slave station C identifiespresence of the “un-transmitted frame at the time of last tokenreception” among the transmission waiting frames stored in thetransmission buffer 64. The data-frame-communication processing unit 24transmits the “un-transmitted frames at the time of last tokenreception” in order of storage in the transmission buffer 64 (in theorder from a frame stored earliest) within the range of the number oftransmittable frames set in the token frame 200. Because, although theframes F8 and F15 are stored in the transmission buffer 64, the“un-transmitted frame at the time of last token reception” is the frame8, only the frame 8 is transmitted. Consequently, the number of framesthat can be transmitted until this token frame 200 returns to the masterstation A is “4 (=5−1)”. Thereafter, the token-frame processing unit 62of the slave station C transmits the token frame 200 in which the numberof transmittable frames is set to “4”, the token repeat counter isincremented by one to “3”, and the master station A is set in thetransmission right acquiring apparatus information. Thedata-frame-communication processing unit 24 of the slave station Cstores, as an un-transmitted frame at the time of last token reception,the frame F15 in the transmission buffer 64 that could not betransmitted at the time of acquisition of the transmission right of thistime. The number-of-transmittable-frames “0” setting station in thetoken frame 200 is not changed and is kept at the value set in thereceived token frame 200. The transmitted frame F8 is erased from thetransmission buffer 64.

Subsequently, when the token-frame processing unit 23 of the masterstation A receives the token frame 200 in which the transmission rightacquiring apparatus information is set to the own station, thetoken-frame processing unit 23 checks the number of transmittable framesin the token frame 200. Because the number of transmittable frames is“4”, the token-frame processing unit 23 determines that allcommunication nodes in the logical ring were able to transmit frames.

Thereafter, as shown in FIG. 7-9, the data-frame-communicationprocessing unit 24 of the master station A performs transmissionprocessing for the frames F9 to F10 stored in the transmission buffer25. Thereafter, the token-frame processing unit 23 transmits the tokenframe 200 in which “4” obtained by subtracting the number of transmittedframes “2” from the value “6” set in advance as the number oftransmittable frames is set in the number of transmittable frames of thetoken frame 200, “0” representing the own station is set in thetransmission permitted station value, the token repeat counter is resetto “1”, and the slave station B is set in the transmission rightacquiring apparatus information. The value of thenumber-of-transmittable-frames “0” setting station in the token frame200 can be kept as it is without being changed or can be erased.Thereafter, the processing explained above is repeatedly executed.Frames that should be transmitted by the communication nodes aretransmitted.

According to the first embodiment, it is determined whether data in atransmission waiting state in the communication nodes can be transmittedin unit of a frame. Therefore, it is unnecessary to calculate an amountof data in the transmission waiting state unlike the past. As a result,it is possible to reduce time after the token frame 200 is receiveduntil data is transmitted. Because the transmission in unit of a frameis performed, when it is determined whether data can be transmittedaccording to a data amount as in the past, if an amount of data thatshould be transmitted in a certain communication node exceeds an amountof data that can be transmitted in the communication system, not all thedata that should be transmitted could not be transmitted. However, inthe first embodiment, it is possible to transmit data in frame unit inthe range of the number of transmittable frames 209.

Specifically, when the token repeat counter 207 is equal to or largerthan the transmission permitted station value 208, it is determinedwhether the own station has the transmission right according to whetherthe number of transmittable frames 209 in the token frame 200 is “0”.Therefore, when there is at least one transmittable frame number 209until the token frame 200 returns to the master station, the slavestation can transmit a frame. As a result, there is an effect thatreal-time communication is possible. There is also an effect that it ispossible to efficiently transmit frames while the token frame 200circulates once.

Further, in the configuration of the logical ring, the token repeatcounter 207 that is incremented by one every time the token frame 200circulates through the slave stations starting from the master stationset as a 0th communication node. Therefore, the slave station canrecognize, with this token repeat counter 207, which slave stationcounted from the master station in the logical ring the slave stationis. Making use of this, a value of the token repeat counter 207 of aslave station in which the number of transmittable frames 209 is set to“0” is stored in the token frame 200 as thenumber-of-transmittable-frames “0” setting station 210. The masterstation sets the number-of-transmittable-frames “0” setting station 210in the transmission permitted station value 208. Therefore, it ispossible to cause a slave station having data that could not betransmitted at the time of last acquisition of the token frame totransmit the data. As a result, there is an effect that data generatedby stations can be transmitted in order from data generated earliest andit is possible to perform communication with importance attached toreal-time properties.

Further, frames that were not able be transmitted at the time ofacquisition of the token frame 200 are stored as last-timeun-transmitted frames. When the token frame 200 is acquired next, onlythe last-time un-transmitted frames are transmitted. Therefore, there isan effect that, even when a frame that should be transmitted isgenerated until the token frame 200 is acquired next after beingacquired last time, it is possible to collectively transmit framessubjected to transmission entry in substantially the same periods in theentire communication system in order from a frame stored in thecommunication node earliest.

Second Embodiment

As in the first embodiment, in a communication system that includes amaster station and slave stations, performs an arithmetic operationusing data from the slave stations, and notifies the slave stations of aresult of the arithmetic operation, it is likely that the datatransmitted by the slave stations concentrate in the master station. Itis also likely that, in the master station, a processing ability oftransmission and reception processing is temporarily more deterioratedthan other processing, a reception buffer is in a full state, and framesare discarded. When such a situation occurs, transmission retryprocessing is carried out to recover discarded data. However, thisrecovery processing leads to deterioration in performance of a network,the master station, and the slave stations. Therefore, in the secondembodiment, a communication system and a communication method that canperform transmission and reception of data without performing therecovery processing when a processing ability of reception processing istemporarily deteriorated in the master station are explained.

The master station according to the second embodiment further has afunction of acquiring, when the token-frame processing unit 23 receivesthe token frame 200 in which the transmission right acquiring apparatusinformation 206 indicates the own station in the first embodiment, thenumber of receivable buffers from a state of use of the reception buffer26 and setting the number of receivable buffers in the number oftransmittable frames 209 of the token frame 200. For example, when amaximum number of stored frames of the reception buffer 26 is n (n is anatural number) and m (m is a natural number, n≧m) frames are stored inthe reception buffer 26 at a point when the token frame 200 is acquired,“n−m” is set in the number of transmittable frames 209 of the tokenframe 200. Components same as those in the first embodiment are denotedby the same reference numerals and signs and explanation of thecomponents is omitted. The configuration of the slave station accordingto the second embodiment is the same as that in the first embodiment.Therefore, explanation of the configuration of the slave station isomitted.

A communication method for data according to the second embodiment isexplained below. FIG. 8 is a flowchart for explaining an example ofprocessing at the time of token frame reception by the master station.First, as at steps S71 to S74 explained with reference to FIG. 6 in thefirst embodiment, the token-frame processing unit 23 of the masterstation transfers the token frame 200 in which the transmission rightacquiring apparatus information 206 does not indicate the own stationand, in the case of the token frame 200 transmitted to the own station,determines whether the number of transmittable frames 209 is “0” (stepsS101 to S104).

When the number of transmittable frames 209 is not “0” (No at stepS104), the token-frame processing unit 23 of the master station acquiresthe number of frames that can be stored in the reception buffer 26 atthe present point (step S105). For example, in a state in which fourframes are stored in the reception buffer 26 that can store up to sixframes, two frames can be stored. The token-frame processing unit 23sets, as the number of transmittable frames 209 of the token frame 200,the acquired number of frames that can be stored in the reception buffer26 (step S106). Subsequently, after the data-frame-communicationprocessing unit 24 transmits frames in the transmission buffer 25 withina range of the set number of transmittable frames 209 (step S107), thetoken-frame processing unit 23 sets, in the number of transmittableframes 209 of the token frame 200, a value obtained by subtracting thenumber of frames transmitted at step S107 from the number oftransmittable frames 209 set at step S106 (step S108). The token-frameprocessing unit 23 sets a value of the token repeat counter 207 to “1”(step S109) and sets the transmission permitted station value 208 to “0”(step S110).

On the other hand, when the number of transmittable frames 209 in thetoken frame 200 received at step S104 is “0” (Yes at step S104), thetoken-frame processing unit 23 of the master station acquires the numberof frames that can be stored in the reception buffer 26 (step S121). Thetoken-frame processing unit 23 sets, as the number of transmittableframes 209 of the token frame 200, the acquired number of frames thatcan be stored in the reception buffer 26 (step S122). The token-frameprocessing unit 23 sets the value of the token repeat counter 207 to “1”(step S123) and sets, in the transmission permitted station value 208, avalue of the number-of-transmittable-frames “0” setting station 210 atthe time of token frame reception (step S124).

Thereafter or after step S110, as at steps S91 and S92 in FIG. 6, thetoken-frame processing unit 23 sets, in the transmission right acquiringapparatus information 206 of the token frame 200, a slave station, whichobtains a transmission right after the own station, acquired from tokencirculation order information (step S125). The token-frame processingunit 23 transmits a token frame set as explained above (step S126) andthe processing ends.

A communication method employing a token frame in such a communicationsystem is explained below with reference to a specific example. FIGS.9-1 to 9-8 are schematic diagrams for explaining an example of aprocedure of a communication method employing a token frame according tothe second embodiment. It is assumed that the master station A canprocess up to six frames. Specifically, it is assumed that the receptionbuffer 26 of the master station A can store up to six frames. Further,it is assumed that the master station A has already performedconfiguration processing for a logical ring, determined the tokencirculation order indicated by (1) in the first embodiment, and notifiedthe slave stations B and C of information concerning the tokencirculation order (token circulation destination information) and,thereafter, the token frame is circulating according to the tokencirculation order.

As shown in FIG. 9-1, it is assumed that, at a point when a token framein which transmission right acquiring apparatus information is set tothe master station A is transmitted from the slave station C, in theslave station B, it is entered from the application layer to the datalink layer to transmit five frames F1 to F5 to the master station A andthese frames F1 to F5 are stored in the transmission buffer 64 in orderas transmission waiting frames.

Thereafter, as shown in FIG. 9-2, when the master station A receives thetoken frame 200 in which the transmission right acquiring apparatusinformation 206 is set to the own station, the master station A checksthe number of transmittable frames in the token frame 200. It is assumedthat the number of transmittable frames is returned as a number that isnot “0”. Therefore, there is no slave station that, although thetransmission right was obtained, was not able to transmit a frameexplained in the first embodiment. The master station A performs normalcommunication for giving the transmission right in order according tothe frame circulation order.

The data-frame-communication processing unit 24 of the master station Acan transmit frames of the transmission buffer 25 because the ownstation obtains the transmission right. However, at this point, becausethere is no frame that should be transmitted, thedata-frame-communication processing unit 24 does not performtransmission processing. Thereafter, the token-frame processing unit 23of the master station A transmits the token frame 200 in which thenumber of transmittable frames is set to the value “6” set in thecommunication system in advance, the token repeat counter is reset to“1”, the transmission permitted station value is set to “0” representingthe own station, and the slave station B is set in the transmissionright acquiring apparatus information 206.

Subsequently, when the token-frame processing unit 62 of the slavestation B receives the token frame 200 in which the transmission rightacquiring apparatus information is set to the own station, thetoken-frame processing unit 62 compares the value of the token repeatcounter in the token frame 200 and the transmission permitted stationvalue and determines whether the own station can transmit data. Becausethe token repeat counter “1”>the transmission permitted station value“0”, the token-frame processing unit 62 recognizes that the slavestation B has obtained a right or transmitting data in a range of thenumber of transmittable frames in the token frame 200.

Thereafter, as shown in FIG. 9-3, the data-frame-communicationprocessing unit 63 of the slave station B transmits the transmissionwaiting frames stored in the transmission buffer 64 in order of storagein the transmission buffer 64 (in order from a frame stored earliest)within the range of the number of transmittable frames set in the tokenframe 200. Because the number of transmission waiting frames F1 to F5stored in the transmission buffer 64 is “5” and the number oftransmittable frames set in the token frame 200 is “6”, thedata-frame-communication processing unit 63 transmits all the frames F1to F5 to the master station A. Consequently, the number of frames thatcan be transmitted until this token frame 200 returns to the masterstation A is “1 (=6−5)”. Thereafter, the token-frame processing unit 62of the slave station B transmits the token frame 200 in which the numberof transmittable frames has been set to “1”, the token repeat counter isincremented by 1 to “2”, and the slave station C is set in thetransmission right acquiring apparatus information. The transmittedframes F1 to F5 are erased from the transmission buffer 64.

Data transmitted from the slave station B is received by the masterstation A and temporarily stored in the reception buffer 26. As aresult, five frames are stored in the reception buffer 26 of the masterstation A. Therefore, the number of reception buffer processable framesis “1”.

Subsequently, the slave station C receives the token frame 200 in whichthe transmission right acquiring apparatus information is set to the ownstation, compares the value of the token repeat counter in the tokenframe 200 and the transmission permitted station value, and determineswhether the own station can transmit data. Because the token repeatcounter “2”>the transmission permitted station value “0”, the slavestation C recognizes that the slave station C has obtained a right fortransmitting data in the range of the number of transmittable frames inthe token frame 200.

Thereafter, as shown in FIG. 9-4, the slave station C attempts totransmit the transmission waiting frames stored in the transmissionbuffer 64 in order of storage in the transmission buffer 64 (in orderfrom a frame stored earliest) within the range of the number oftransmittable frames “1” set in the token frame 200. However, becausethere is no data that should be transmitted in the transmission buffer64, eventually, the slave station C does not transmit a frame.

The token-frame processing unit 62 of the slave station C transmits thetoken frame 200 in which the token repeat counter is incremented by oneto “3” and the master station A is set in the transmission rightacquiring apparatus information. In this token frame 200, the number oftransmittable frames remains “1”.

It is assumed that, in time until the transmission of the token frame200 by the slave station C, in the master station A, processing of oneframe among the frames stored in the reception buffer 26 has ended and astorage capacity for two frames has been secured in the receptionbuffer. In other words, it is assumed that the number of receptionbuffer processable frames of the master station A has increased to “2”.

It is assumed that, thereafter, as shown in FIG. 9-5, in a stateimmediately after the slave station C transmits the token frame 200 inwhich the transmission right acquiring apparatus information has beenset to the master station A, in the slave station C, it is entered fromthe application layer to the data link layer to transmit the frames F6to F9 to the master station A and these frames F6 to F9 are stored inthe transmission buffer 64 as transmission waiting frames.

Subsequently, as shown in FIG. 9-6, when the master station A receivesthe token frame 200 in which the transmission right acquiring apparatusinformation has been set to the own station, the master station A checksthe number of transmittable frames in the token frame 200. Because thenumber of transmittable frames 209 is “1”, the master station Adetermines that all the communication nodes in the logical ring wereable to transmit frames. As a result, the master station A can performtransmission processing for frames stored in the transmission buffer 25.However, in this example, because no frame is stored in the transmissionbuffer 25, the transmission processing for frame by the master station Ais not performed.

The master station A originally sets a value “6”, which is set in thecommunication system in advance, as the number of transmittable framesof the token frame 200. However, because the number of reception bufferprocessable frames is “2”, the master station A transmits the tokenframe 200 in which “2” has been set in the number of transmittableframes of the token frame 200, the token repeat counter has been resetto “1”, and the slave station B has been set in the transmission rightacquiring apparatus information.

Subsequently, when the slave station B receives the token frame 200 inwhich the transmission right acquiring apparatus information has beenset to the own station, the slave station B compares the value of thetoken repeat counter in the token frame 200 and the transmissionpermitted station value and determines whether the own station cantransmit data. Because the token repeat counter “1”>the transmissionpermitted station value “0”, the slave station B recognizes that theslave station B has obtained a right for transmitting data in the rangeof the number of transmittable frames in the token frame. However,because no frame is present in the transmission buffer 64 of the slavestation B, the slave station B does not perform transmission processingfor frames.

Thereafter, as shown in FIG. 9-7, the slave station B transmits thetoken frame 200 in which the token repeat counter has been incrementedby one to “2” and the slave station C has been set in the transmissionright acquiring apparatus information. The number of transmittableframes in the token frame 200 is not changed and is kept at a value setin the received token frame 200.

Subsequently, the slave station C receives the token frame 200 in whichthe transmission right acquiring apparatus information is set to the ownstation, compares the value of the token repeat counter in the tokenframe 200 and the transmission permitted station value, and determineswhether the own station can transmit data. Because the token repeatcounter “2”>the transmission permitted station value “0”, the slavestation C recognizes that the slave station C has obtained a right fortransmitting data in the range of the number of transmittable frames inthe token frame.

Thereafter, as shown in FIG. 9-8, the data-frame-communicationprocessing unit 63 of the slave station C transmits the transmissionwaiting frames stored in the transmission buffer 64 in order of storagein the transmission buffer 64 (in order from a frame stored earliest)within the range of the number of transmittable frames 209 set in thetoken frame 200. The frames F6 to F9 have been stored in thetransmission buffer 64. However, because the number of transmittableframes is “2”, the data-frame-communication processing unit 63 transmitsonly the frames F6 and F7. Consequently, the number of frames that canbe transmitted until this token frame 200 returns to the master stationA is “0 (=2−2)”. Thereafter, the token-frame processing unit 62 of theslave station C transmits the token frame 200 in which the number oftransmittable frames has been set to “0”, “2”, which is a value of thetoken repeat counter at the time when a token frame is received(represents the own station), has been set in thenumber-of-transmittable-frames “0” setting station, the token repeatcounter is incremented by one to “3”, and the master station A has beenset in the transmission right acquiring apparatus information. The slavestation C stores, as un-transmitted frames at the time of the number oftransmittable frames “0”, the frames F8 and F9 in the transmissionbuffer 64 that were not able to be transmitted at the time ofacquisition of the transmission right of this time. The transmittedframes F6 and F7 are erased from the transmission buffer 64. Thereafter,the processing explained above or the processing explained in the firstembodiment is performed.

As explained above, the number of transmittable frames is variably setaccording to the reception processing ability of the master station Aand transmission processing for frames corresponding to the setting isperformed.

According to the second embodiment, the number of frames that can betransmitted when the next token is circulated is set according to thereception processing ability of the master station. Therefore, it ispossible to prevent data from concentrating on the master station andprevent frames from being discarded because the reception buffer isfull. Because recovery processing by the discarding of frames is notperformed, there is also an effect that it is possible to preventdeterioration in communication performance and data processingperformance in the master station, the slave stations, and thecommunication system including these communication nodes.

Third Embodiment

For example, in an FA system in which a programmable controllercorresponding to the master station and control target apparatusescorresponding to the slave stations are connected by a network,arithmetic processing is performed by the programmable controller basedon data output from the control target apparatuses and a result of thearithmetic operation is output to the control target apparatuses toperform control of the control target apparatuses. Therefore, real-timeproperties of data are requested. In such a case, when high accuracy isrequired for the result of the arithmetic processing, a large amount ofdata from the control target apparatuses are required. When very highaccuracy is not required for the result of the arithmetic processing,the amount of data from the control target apparatus can be small. Inthis way, in the FA system, there are degrees of importance concerningthe control target apparatuses.

Therefore, when a load of transmission requests from the slave stationstemporarily increases, if reception processing for data from all theslave stations is performed by the method explained in the firstembodiment, it is likely that data from the control target apparatuseshaving high degrees of importance cannot be easily received andhighly-accurate arithmetic processing cannot be performed. Therefore, inthe third embodiment, a communication system and a communication methodthat can preferentially transmit data from slave stations having highdegrees of importance to the master station are explained.

FIG. 10 is a schematic block diagram of a functional configuration of amaster station according to the third embodiment. This master stationfurther includes, in the configurations of the first and secondembodiments, achanged-destination-information-during-priority-processing storing unit27 that stores changed destination information during priorityprocessing that indicates to which slave station a transmission right ispreferentially given when a load of transmission requests of slavestations increases and a number-of-transmittable-frames “0”setting-station storing unit 28 that stores thenumber-of-transmittable-frames “0” setting station 210 in the tokenframe 200 when a load of transmission requests of the slave stationsincreases. The token-frame processing unit 23 determines whether or nota load of transmission requests of the slave stations has increasedaccording to whether or not the number of transmittable frames 209 inthe token frame 200 is “0”.

The changed destination information during priority processing stored inthe changed-destination-information-during-priority-processing storingunit 27 is set to a slave station having an earliest token circulationturn among slave stations with a priority level set highest in a logicalring (token circulation order). The priority levels of the slavestations have been set in advance according to types of the slavestations or set by a user.

FIG. 11 is a schematic diagram of a relation between station numbers andsequence numbers of communication nodes included in a communicationsystem. The station numbers are information arbitrarily set by the userto uniquely identify the communication nodes of the communicationsystem. The sequence numbers are numbers that indicate order ofarrangement of the slave stations with the master station A set as areference in the logical ring as explained in the first embodiment. Themaster station A stores the station numbers and the sequence numbers inassociation with each other in, for example, the token circulation orderinformation of the token-circulation-order-information storing unit 22.Information concerning a priority level of a communication node havingwhich station number is to be set high by the user is stored in thechanged-destination-information-during-priority-processing storing unit27 as changed destination information during priority processing. Forexample, if a priority level of the slave stations C and E is thehighest, the slave station C is selected as the changed destinationinformation during priority processing. This is because, by selectingthe slave station C, a token frame circulates to the slave station Ehaving the high priority level in a later logical ring. Informationconcerning the station numbers set in the communication nodes isacquired from the communication nodes during logical ring configurationprocessing by the logical-ring configuring unit 21, whereby the relationshown in FIG. 11 can be obtained as association between the stationnumbers and the sequence numbers of the communication nodes.

When the token-frame processing unit 23 receives the token frame 200 andthe number of transmittable frames of the received token frame is “0”,the token-frame processing unit 23 stores, in thenumber-of-transmittable-frames “0” setting-station storing unit 28, avalue of the number-of-transmittable-frames “0” setting stations storedin the token frame 200 and sets, in the transmission permitted stationvalue of the token frame 200, the changed destination information duringpriority processing stored in thechanged-destination-information-during-priority-processing storing unit27. When the token-frame processing unit 23 receives the token frame 200in which the transmission right acquiring apparatus information is setto the own station in a state in which thenumber-of-transmittable-frames “0” setting station is stored in thenumber-of-transmittable-frames “0” setting-station storing unit 28, thetoken-frame processing unit 23 generates the token frame 200 in whichthe number-of-transmittable-frames “0” setting station stored in thenumber-of-transmittable-frames “0” setting-station storing unit 28 isset in the transmission permitted station value and erases informationin the number-of-transmittable-frames “0” setting-station storing unit28.

Components same as those explained in the first and second embodimentsare denoted by the same reference numerals and signs and explanation ofthe components is omitted. The configuration of the slave station is thesame as that explained in the first and second embodiments. Therefore,explanation of the configuration is also omitted.

A processing procedure by the master station for giving a datatransmission right to a slave station desired to be given priority isexplained below. FIGS. 12-1 and 12-2 are flowcharts for explaining anexample of a processing procedure for a token frame of the masterstation according to the third embodiment. First, as at steps S71 to S73explained with reference to FIG. 6 in the first embodiment, thetoken-frame processing unit 23 of the master station transfers the tokenframe 200 in which the transmission right acquiring apparatusinformation 206 does not indicate the own station (steps S151 to S153).

In the case of the token frame 200 in which the transmission rightacquiring apparatus information 206 indicates the own station, thetoken-frame processing unit 23 determines whether information is storedin the number-of-transmittable-frames “0” setting-station storing unit28 (step S154). When information is not stored in thenumber-of-transmittable-frames “0” setting-station storing unit 28 (Noat step S154), the token-frame processing unit 23 recognizes that thelast circulation of the token frame 200 is not processing for causing acommunication node having a high priority level to perform transmissionof data (hereinafter referred to as processing of a priority mode) andthe token frame 200 for equally giving the data transmission right tothe communication nodes in the communication system explained in thefirst and second embodiments has been issued. Therefore, subsequently,the token-frame processing unit 23 determines whether the number oftransmittable frames 209 in the token frame 200 is “0” (step S155).

When the number of transmittable frames 209 is not “0” (No at stepS155), the token-frame processing unit 23 determines that a load oftransmission requests of the communication nodes included in thecommunication system has not increased. Thereafter, the token-frameprocessing unit 23 sets the number of transmittable frames to a valueset in advance (step S156). The data-frame-communication processing unit24 transmits a frame in the transmission buffer 25 (step S157). Thetoken-frame processing unit 23 sets, in the number of transmittableframes 209 of the token frame 200, a value obtained by subtracting thenumber of frames transmitted at step S157 from the number oftransmittable frames set at step S156 (step S158). The token-frameprocessing unit 23 resets “1” in the token repeat counter 207 (stepS159) and sets “0” in the transmission permitted station value 208 ofthe token frame 200 (step S160). The token-frame processing unit 23 setsa communication node conforming to the token circulation orderinformation in the transmission right acquiring apparatus information206 (step S175) and transmits the token frame 200 (step S176) and theprocessing ends.

On the other hand, when the number of transmittable frames is 0 at stepS155 (Yes at step S155), the token-frame processing unit 23 determinesthat a load of transmission requests of the communication nodes includedin the communication system has temporarily increased. Thereafter, thetoken-frame processing unit 23 sets the number of transmittable framesto a value set in advance (step S171) and resets “1” in the token repeatcounter 207 (step S172). The token-frame processing unit 23 stores, inthe number-of-transmittable-frames “0” setting-station storing unit 28,the number-of-transmittable-frames “0” setting station 210 in thereceived token frame 200 (step S173) and sets, in the transmissionpermitted station value 208, a value stored in the changed destinationinformation during priority processing of thechanged-destination-information-during-priority-processing storing unit27 (step S174). Thereafter, the processing at step S175 and subsequentsteps is performed.

When information is stored in the number-of-transmittable-frames “0”setting-station storing unit 28 at step S154 (Yes at step S154), themaster station recognizes that the processing in the priority mode wasperformed at the time of the last circulation of the token frame 200.Therefore, the token-frame processing unit 23 of the master station setsthe number of transmittable frames to a value set in advance (stepS181). The data-frame-communication processing unit 24 transmits a framein the transmission buffer 25 (step S182). The token-frame processingunit 23 sets, in the number of transmittable frames 209 of the tokenframe 200, a value obtained by subtracting the number of framestransmitted at step S182 from the number of transmittable frames set atstep S181 (step S183). The token-frame processing unit 23 resets thetoken repeat counter 207 in the token frame 200 to “1” (step S184),sets, in the transmission permitted station value 208, thenumber-of-transmittable-frames “0” setting station stored in thenumber-of-transmittable-frames “0” setting-station storing unit 28 (stepS185), and sets a communication node conforming to the token circulationorder information in the transmission right acquiring apparatusinformation 206 (step S186). The token-frame processing unit 23transmits the token frame 200 in which the various kinds of informationin the data 204 have been set as explained above (step S187). At thispoint, the token-frame processing unit 23 erases thenumber-of-transmittable-frames “0” setting station stored in thenumber-of-transmittable-frames “0” setting-station storing unit 28.Consequently, the processing ends.

A specific example of transmission processing for a token frameaccording to the third embodiment is explained below. FIGS. 13-1 to 13-5are schematic diagrams of an example of the transmission processing fora token frame according to the third embodiment. This communicationsystem is configured by connecting one master station A and three slavestations B to D via the switching hub 101. It is assumed that tokencirculation order has been determined as indicated by (2) below by thelogical-ring configuring unit 21 of the master station A. It is assumedthat, after the master station A notifies the slave stations B to D ofinformation concerning the token circulation order indicated by (2)below (token circulation destination information), the token frame 200circulates in the communication system according to the tokencirculation order.Master station A→slave station B→slave station C→slave station D→masterstation A  (2)

It is assumed that a priority level of data transmitted from the slavestation B is high and “2”, which indicates the slave station B as thechanged destination information during priority processing is set in thechanged-destination-information-during-priority-processing storing unit27. It is assumed that the reception buffer 26 of the master station Acan store up to six frames.

First, as shown in FIG. 13-1, the slave station C has acquired atransmission right in which the number of transmittable frames is “3”and transmits the frames F1 to F3 among the transmission waiting framesF1 to F5 of the transmission buffer 64. The slave station C transmitsthe token frame 200 in which the number of transmittable frames has beenset to “0”, the number-of-transmittable-frames “0” setting station 210has been set to “2”, which is a sequence number of the own station, thetoken repeat counter has been set to “3”, and the transmission rightacquiring apparatus information has been set to the slave station D. Theslave station C stores, as un-transmitted frames at the time of numberof transmittable frames “0” setting, the frames F4 and F5 in thetransmission buffer 64 that were not able to be transmitted.

Subsequently, as shown in FIG. 13-2, the slave station D acquires thetransmission right as explained in the first embodiment. However, theslave station D cannot transmit the un-transmitted frames F6 and F7 inthe transmission buffer 64. Therefore, the slave station D transmits thetoken frame 200 in which the token repeat counter 207 has been set to“4” and the transmission right acquiring apparatus information has beenset to the master station A. The slave station D stores, asun-transmitted frames at the time of last token reception, the frames F6and F7 in the transmission buffer 64 that have not been able to betransmitted.

It is assumed that, at a point immediately after the token frame 200 inwhich the transmission right acquiring apparatus information has beenset to the master station A is transmitted from the slave station D, inthe slave station B, it is entered from the application layer to thedata link layer to transmit six frames F8 to F13 to the master station Aand these frames F8 to F13 are stored in the transmission buffer 64 inorder as transmission waiting frames. Further, it is assumed that, inthe slave station C, it is entered from the application layer to thedata link layer to transmit two frames F14 and F15 to the master stationA and these frames F14 and F15 are stored in the transmission buffer 64in order as transmission waiting frames. It is assumed that, at thispoint, in the master station A, data is stored in the reception buffer26 to the full storage capacity by the frames F1 to F3 received from theslave station C.

In such a state, when the token-frame processing unit 23 of the masterstation A receives the token frame 200 in which the transmission rightacquiring apparatus information has been set to the own station, thetoken-frame processing unit 23 checks whether information is stored inthe number-of-transmittable-frames “0” setting-station storing unit 28.It is assumed that information is not stored in thenumber-of-transmittable-frames “0” setting-station storing unit 28.Thereafter, the token-frame processing unit 23 further checks the numberof transmittable frames in the token frame 200. Because the number oftransmittable frames is “0”, the token-frame processing unit 23 shiftsto the priority mode for preferentially transmitting data of the slavestation B set in advance in the changed destination information duringpriority processing of thechanged-destination-information-during-priority-processing storing unit27.

As shown in FIG. 13-3, the master station A stores, in thenumber-of-transmittable-frames “0” setting-station storing unit 28, thenumber-of-transmittable-frames “0” setting station 210 in the receivedtoken frame 200. Subsequently, the master station A transmits the tokenframe 200 in which the number of transmittable frames is set to thevalue “6” set in advance in the communication system, the token repeatcounter has been reset to “1”, the transmission permitted station valuehas been set to the value “1” (=the slave station B) stored in thechanged destination information during priority processing, and theslave station B has been set in the transmission right acquiringapparatus information.

Thereafter, the slave station B receives the token frame 200 in whichthe transmission right acquiring apparatus information 206 has been setto the own station and compares the value of the token repeat counter207 in the token frame 200 and the transmission permitted station value208. Because the token repeat counter “1”=the transmission permittedstation value “1”, the slave station B recognizes that the transmissionright has been obtained.

As shown in FIG. 13-4, the slave station B transmits the six frames F8to F13 within a range specified by the number of transmittable framesamong the transmission waiting frames stored in the transmission buffer64. Thereafter, the slave station B transmits the token frame 200 inwhich the token repeat counter has been incremented by one to “2”, thenumber of transmittable frames has been set to “0”, and the transmissionright acquiring apparatus information has been set to the slave stationC. It is assumed that, while data is transmitted in the priority mode,even if the number of transmittable frames is “0”, setting (writing) inthe number-of-transmittable-frames “0” setting station in the tokenframe 200 is not performed.

In the method in the first embodiment, the master station A sets theinformation in the data of the token frame 200 to give the priorityright to the slave station C set in the number-of-transmittable-frames“0” setting station in FIG. 13-2. However, in the third embodiment, whenthe number of processable frames of the reception buffer 26 of themaster station A is “0”, the transmission right is given to the slavestation B stored in thechanged-destination-information-during-priority-processing storing unit27.

Subsequently, the token frame 200 circulates to the slave stations C andD in order. However, because the number of transmittable frames is “0”in both the slave stations C and D, a data frame cannot be transmitted.As shown in FIG. 13-5, the slave station D transmits the token frame 200in which the token repeat counter has been set to “4”, the number oftransmittable frames has been set to “0”, and the transmission rightacquiring apparatus information has been set to the master station A.

Thereafter, as shown in FIG. 13-5, the master station A receives thetoken frame 200 in which the transmission right acquiring apparatusinformation has been set to the own station and checks whetherinformation is stored in the number-of-transmittable-frames “0”setting-station storing unit 28. Because information has been stored,the token-frame processing unit 23 of the master station A sets, in thetransmission permitted station value of the token frame 200, the value“2” stored in the number-of-transmittable-frames “0” setting-stationstoring unit 28. The token-frame processing unit 23 generates andtransmits the token frame 200 in which the token repeat counter has beenreset to “1”, the number of transmittable frames is set to “6”, and “1”indicating the slave station B has been set in the transmission rightacquiring apparatus information. When generation processing for thetoken frame 200 ends, the token-frame processing unit 23 erases theinformation stored in the number-of-transmittable-frames “0”setting-station storing unit 28.

Thereafter, processing same as the processing explained in the firstembodiment is performed. Specifically, first, the transmission right isgiven to the slave station C, the frames F4 and F5 stored asun-transmitted frames at the time of number of transmittable frames “0”setting in the transmission buffer 64 are transmitted, and the frames F6and F7 stored as un-transmitted frames at the time of last tokenreception in the transmission buffer 64 of the slave station D aretransmitted.

In the example explained above, a load of communication requests of theslave stations temporarily increases. However, the processing explainedabove is not performed only in this case. It is also possible to executethe same processing for shifting the communication system to thepriority mode, for example, when reception processing of the masterstation has been deteriorated.

According to the third embodiment, when the number of transmittableframes of a received token frame is “0”, the transmission right is givento a slave station having a high priority level set in advance, contentsof the number-of-transmittable-frames “0” setting station 210 aretemporarily stored, and the transmission right is not given to the slavestations set in the number-of-transmittable-frames “0” setting station210. As a result, there is an effect that it is possible to transmit,while maintaining real-time properties, data of the slave stations thatneed to frequently transmit data to the master station.

Fourth Embodiment

In a fourth embodiment, another method of giving a transmission right toa slave station having the highest priority level among slave stationsin a communication system when the number of transmittable frames of areceived token frame is “0”.

FIG. 14 is a diagram of an example of a format of a token frame used inthe fourth embodiment. In this token frame 200, the transmissionpermitted station value 208 shown in FIG. 3 includes a starttransmission permitted station value 208 a and an end transmissionpermitted station value 208 b for specifying a range on a logical ringof the slave stations that are caused to preferentially perform datatransmission. Specifically, when the number of transmittable frames ofthe received token frame is “0” and the communication system shifts tothe priority mode, the start transmission permitted station value 208 aindicates a sequence number of a first communication node to which thetransmission right is preferentially given among communication nodes onthe logical ring. The end transmission permitted station value 208 bindicates a sequence number of the last communication node to which thetransmission right is preferentially given among the communication nodeson the logical ring. As a result, among the communication nodes on thelogical ring, communication nodes from the communication node having thesequence number of the start transmission permitted station value 208 ato the communication node having the sequence number of the endtransmission permitted station value 208 b obtain the transmission rightand perform transmission of data frames.

The configuration of the master station is different from theconfiguration in the third embodiment in that changed destinationinformation during priority processing including a start transmissionright permitted station value and an end transmission right permittedstation value in shifting to the priority mode is stored in thechanged-destination-information-during-priority-processing storing unit27. Otherwise, the configuration of the master station is the same asthe configuration in the third embodiment. Therefore, explanation of theconfiguration is omitted.

The configuration of the slave station is different from theconfigurations in the first to third embodiments in that, after thetoken-frame processing unit 62 receives the token frame 200 in which thetransmission right acquiring apparatus information 206 indicates the ownstation, the token-frame processing unit 62 determines whether a valueof the token repeat counter 207 is equal to or larger than the starttransmission permitted station value 208 a and equal to or smaller thanthe end transmission permitted station value 208 b. Otherwise, theconfiguration of the slave station is the same as the configuration inthe first to third embodiments. Therefore, explanation of theconfiguration is omitted.

A communication method in the communication system in such aconfiguration is the same as the communication method explained in thethird embodiment. Therefore, explanation of the communication method isalso omitted.

According to the fourth embodiment, when the number of transmittableframes of the received token frame is “0”, a start sequence number andan end sequence number on the logical ring of the communication nodesdesired to be caused to preferentially transmit data are designated.Therefore, there is an effect that it is possible to cause a set ofarbitrarily selected communication nodes on the logical ring topreferentially transmit data. The data transmission processing inshifting to the priority mode is performed only by the communicationnodes in the range set by the start transmission permitted station valueand the end transmission permitted station value. Therefore, comparedwith the third embodiment, it is possible to reduce processing performedwhile the token frame circulates once.

In the third and fourth embodiments, a priority-processing-informationsetting unit can be provided in the master station. A slave stationinput by a user and desired to be given priority can be set in thechanged-destination-information-during-priority-processing storing unit27. Consequently, it is possible to perform, according to an occurredevent, control of the transmission right of the slave station desired tobe given priority.

In the first to fourth embodiments, the size of a frame transmitted by acommunication node that has obtained the transmission right is notspecifically limited. However, it is also possible to set the masterstation to transmit only frames equal to or shorter than predeterminedlength. When a frame longer than the set predetermined length istransmitted, data only has to be divided into transmittable length in alayer higher than the data link layer. When the data is received, areceiving side only has to collect the divided data into one.

Fifth Embodiment

The communication system according to the first to fourth embodimentscan be applied to, for example, an FA network (an FA system) including acontrol target apparatus and a control apparatus such as a programmablecontroller that performs a predetermined arithmetic operation using astate of the control target apparatus as input data, and outputs anoperation condition for the control target apparatus as output data.

In the FA network, synchronous data (cyclic data) always transmittedfrom communication nodes at the time of token frame reception andasynchronous data (un-cyclic data) that do not have to be periodicallytransmitted because real-time properties are not regarded as importantcoexist. Examples of the synchronous data include control data such asdata for a master station to control slave stations and data acquired bythe slave stations necessary for an arithmetic processing by the masterstation. Examples of the asynchronous data include data such as acontrol log of the slave stations.

In such a network, the functions explained in the first to fourthembodiments do not have to be applied to all data transmitted in thecommunication system. Specifically, when a communication node receives atoken frame in which the own station is set in transmission rightacquiring apparatus information, the communication node always transmitsthe synchronous data and transmits the asynchronous data according tothe method explained in the first to fourth embodiments.

In this case, it is sufficient that a flag for determining whether aframe is a synchronous frame or an asynchronous frame (an asynchronousframe determining flag) is provided in the frame, a communication nodethat has acquired the transmission right reads asynchronous framedetermining flag information of the frame, applies the first to fourthembodiments only to the asynchronous frame, and always transmits thesynchronous frame when the transmission right is acquired.

According to the fifth embodiment, there is an effect that it ispossible to change, according to the importance of data, a target forwhich real-time properties are secured.

The communication methods in the master station and the slave stationcan be realized by executing computer programs, in which processingprocedures of the respective communication methods are written, with acomputer such as a programmable controller or a personal computer havinga CPU (central processing unit). In this case, the CPU (control means)of the computer executes the processing steps of the communicationmethods explained above according to the computer programs. Thesecomputer programs are recorded in a computer-readable recording mediumsuch as a hard disk, a floppy disk, a CD (Compact Disk)-ROM (Read OnlyMemory), an MO (Magneto-Optical disk), or a DVD (Digital Versatile Diskor digital Video Disk) and executed by being read out from the recordingmedium by the computer. These computer programs can also be distributedvia a network (a communication line) such as the Internet.

Further, the master station can be a communication management circuit inwhich the processing units described in the embodiments are realized bya circuit that executes processing according to the processing procedureexplained above. Similarly, the slave circuit can also be acommunication circuit in which the processing units described in theembodiments are realized by a circuit that executes processing accordingto the processing procedure explained above.

Furthermore, the master station can also be an LSI (Large-ScaleIntegration) in which the processing units described in the embodimentsare manufactured to execute processing according to the processingprocedure. Similarly, the slave station can also be an LSI in which theprocessing units described in the embodiments are manufactured toexecute processing according to the processing procedure.

INDUSTRIAL APPLICABILITY

As explained above, the communication management apparatus according tothe present invention is useful for a communication apparatus thatmanages a data transmission right in a network system connected by anEthernet in which real-time properties of data communication isrequested.

The invention claimed is:
 1. A communication management apparatus thatmanages transmission of data in a network in which a plurality ofcommunication nodes are connected by an Ethernet, the communicationmanagement apparatus comprising: a token-frame receiving unit thatreceives a token frame for acquiring a transmission right of data anddetermines whether the transmission right is acquired by analyzing thetoken frame; a token-frame transmitting unit that transmits the tokenframe which includes (i) first transmission right acquisitiondetermination information indicating a sequence number of acommunication node that can acquire the transmission right in acirculation order of the token frame in the network, (ii) secondtransmission right acquisition determination information indicating aspecific number of frames that can be transmitted while the token framecirculates once, wherein the specific number is variable based onsubtracting a number of transmitted data frames from a previously setnumber of transmittable frames, and (iii) information on anothercommunication node that subsequently acquires the transmission right;and a data-frame-communication processing unit that performs receptionprocessing for data frames from other communication nodes and, when thetransmission right is acquired, transmits data in a frame unit, whereinthe first transmission right acquisition determination informationincludes (i) a token circulation number indicating the sequence numbercounted from the communication management apparatus in the circulationorder of the token frame in the network and (ii) a transmissionpermitted station value indicating the sequence number in thecirculation order of the communication node that can transmit a dataframe, and the token-frame transmitting unit sets the token circulationnumber to a value of “1” and sets, in the transmission permitted stationvalue, a value corresponding to information in the received token frame.2. The communication management apparatus according to claim 1, whereinthe token-frame transmitting unit sets the transmission permittedstation value, indicating the sequence number of the own station, to avalue of “0” when the second transmission right acquisitiondetermination information of the received token frame is not “0”.
 3. Thecommunication management apparatus according to claim 1, wherein in thereceived token frame, when the second transmission right acquisitiondetermination information is “0”, and a number of transmittable framesis set to a value of “0” in a setting station information, whichindicates a communication apparatus that sets the second transmissionright acquisition determination information to a value of “0”, thetoken-frame receiving unit does not give a transmission instruction forthe data frame to the data-frame-communication processing unit, and thetoken-frame transmitting unit sets the number of transmittable frames toa value of “0” in the setting station information in the transmissionpermitted station value of the token frame.
 4. The communicationmanagement apparatus according to claim 3, further comprising anumber-of-transmittable-frames “0” setting-station storing unit thatstores contents of the number-of-transmittable-frames “0”setting-station information, wherein the token-frame-receiving unitstores, when the token-frame receiving unit receives the token frame inwhich the transmission right acquiring apparatus information indicatesthe own apparatus and the second transmission right acquisitiondetermination information is “0”, contents of thenumber-of-transmittable-frames “0” setting station information in thenumber-of-transmittable-frames “0” setting-station storing unit, and thetoken-frame transmitting unit sets, in the transmission permittedstation value, the sequence number of a communication apparatus to whichthe transmission right is preferentially given when the secondtransmission right acquisition determination information is “0”.
 5. Thecommunication management apparatus according to claim 4, wherein thetoken-frame transmitting unit sets, when the second transmission rightacquisition determination information of the received token frame is“0”, a start transmission permitted station value and an endtransmission permitted station value indicating the sequence number in arange of the communication apparatus to which the transmission right ispreferentially given in the transmission permitted station value of thetoken frame.
 6. The communication management apparatus according toclaim 4, wherein, when the number of transmittable frames “0” settingstation information is stored in the number-of-transmittable-frames “0”setting-station storing unit, the token-frame transmitting unit sets,when the token-frame transmitting unit receives the token frame in whichthe transmission right acquiring apparatus information indicates the ownapparatus, the number of transmittable frames “0” setting stationinformation in the transmission permitted station value of the tokenframe and erases the number of transmittable frames “0” setting stationinformation stored in the number-of-transmittable-frames “0”setting-station storing unit.
 7. A communication node that receives, ina network in which a plurality of communication nodes are connected byan Ethernet, a token frame for acquiring a transmission right of datacirculated according to a circulation order determined by acommunication management apparatus and performs transmission of data,the communication node comprising: atoken-circulation-destination-information storing unit that stores tokencirculation destination information indicating the communication nodethat acquires the transmission right of data next based on thecirculation order; a token-frame receiving unit that receives the tokenframe, determines whether the transmission right is acquired byanalyzing the token frame, and determines, when the transmission rightis acquired, whether transmission of a data frame is possible usingfirst transmission right acquisition determination informationindicating a sequence number of a communication apparatus that canacquire the transmission right in the circulation order and secondtransmission right acquisition determination information indicating aspecific number of transmittable frames at the time of the token framereception that are stored in the received token frame, wherein thespecific number is variable based on subtracting a number of transmitteddata frames from a previously set number of transmittable frames; atoken-frame transmitting unit that transmits the token frame in which avalue obtained by subtracting, when transmission of the data frame ispossible, a number of transmitted data frames from the secondtransmission right acquisition determination information is set in thesecond transmission right acquisition determination information; and adata-frame-communication processing unit that performs receptionprocessing for data frames from the other communication nodes and, whenthe transmission right is acquired, transmits data in a frame unit,wherein the first transmission right acquisition determinationinformation includes a token circulation number indicating the sequencenumber counted from the communication management apparatus in thecirculation order and a transmission permitted station value indicatingthe sequence number in the circulation order of a communication nodethat can transmit a data frame, and when it is determined that thetransmission right is acquired, the token-frame receiving unitdetermines whether transmission of the data frame is possible using thetoken circulation number and the transmission permitted station value.8. The communication node according to claim 7, wherein the token-frametransmitting unit sets, as a new version of the token circulationnumber, the token circulation number of the received token frameincremented by one.
 9. The communication node according to claim 7,wherein when the token circulation number in the token frame is equal toor larger than the transmission permitted station value and the secondtransmission right acquisition determination information is larger than“0”, the token-frame receiving unit determines that transmission of adata frame is possible, and the data-frame-communication processing unittransmits the data frame within a range of the second transmission rightacquisition determination information.
 10. The communication nodeaccording to claim 9, wherein the token frame stores number oftransmittable frames “0” setting station information indicating acommunication node that sets the second transmission right acquisitiondetermination information to “0”, and when a value obtained bysubtracting a number of transmitted data frames from second transmissionright acquisition determination information in a received token framechanges to “0”, the token-frame transmitting unit sets a value of thetoken circulation number in the received token frame in the number oftransmittable frames “0” setting station information.
 11. Acommunication method for data employing a token frame in a communicationsystem that manages transmission of data in a network in which aplurality of communication nodes are connected by an Ethernet cable andincludes a communication management apparatus that is one of thecommunication nodes and communication apparatuses that are the othercommunication nodes in the network, the communication method comprising:a transmission right determining step for determining, by thecommunication management apparatus, whether a communication node in theplurality of communication nodes indicated by transmission rightacquiring apparatus information that acquires a transmission right nextin the received token frame is the own apparatus; a first data frametransmitting step for transmitting, by the communication managementapparatus, when the transmission right acquiring apparatus informationindicates the own apparatus, data in a transmission waiting state in aframe unit; a first token frame transmitting step for transmitting, bythe communication management apparatus, the token frame which includes(i) first transmission right acquisition determination informationindicating a sequence number of a communication apparatus that canacquire the transmission right in token circulation order forcirculating the token frame in the network, (ii) second transmissionright acquisition determination information indicating a specific numberof frames that can be transmitted while the token frame circulates oncefrom the own station, wherein the specific number is variable based onsubtracting a number of transmitted data frames from a previously setnumber of transmittable frames, and (iii) transmission right acquiringapparatus information that is a next transmission destination of thetoken frame after the own apparatus acquired from the token circulationorder; a token-frame-acquisition determining step for determining, bythe communication apparatus, whether the transmission right acquiringapparatus information of the received token frame indicates the ownapparatus; a first transmission right determining step for determining,by the communication apparatus, when the transmission right acquiringapparatus information indicates the own apparatus, using the firsttransmission right acquisition determination information, whether theown station has the transmission right; a second transmission rightdetermining step for determining, by the communication apparatus, whenit is determined in the first transmission right determining step thatthe own station has the transmission right, possibility of transmissionof data frames using the second transmission right acquisitiondetermination information; a second data frame transmitting step fortransmitting, by the communication apparatus, when it is determined thatthe data frames can be transmitted in the second transmission rightdetermining step, the data frames within a range of the secondtransmission right acquisition determination information; and a secondtoken frame transmitting step for setting, by the communicationapparatus, after it is determined in the first transmission rightdetermining step that the own station does not have the transmissionright, after it is determined in the second transmission rightdetermining step that the data frames cannot be transmitted, or afterthe data frames are transmitted in the data frame transmitting step, inthe transmission right acquiring apparatus information, tokencirculation destination information indicating the communication node inthe plurality of communication nodes to which the transmission right isgiven after the own station, and transmitting, when the data frames aretransmitted in the second data frame transmitting step, the token framein which a value obtained by subtracting a number of the data framesfrom the second transmission right acquisition determination informationat the time of the token frame reception has been set in the secondtransmission right acquisition determination information, wherein thefirst transmission right acquisition determination information includesa token circulation number indicating the sequence number based on thecommunication management apparatus in the token circulation order of acommunication apparatus set in the transmission right acquiringapparatus information and a transmission permitted station valueindicating a sequence number in the token circulation order of acommunication apparatus that can transmit a data frame in the dataframes, in the first token frame transmitting step, the communicationmanagement apparatus sets “1” in the token circulation number and sets,in the transmission permitted station value, a value corresponding toinformation set in the received token frame, in the first transmissionright determining step, the communication apparatus determines presenceor absence of the transmission right using the token circulation numberand the transmission permitted station value, and in the second tokenframe transmitting step, the communication apparatus sets, in the tokencirculation number, the sequence number indicating the token circulationdestination information.
 12. The communication method according to claim11, wherein, in the second token frame transmitting step, thecommunication apparatus sets, as a new version of the token circulationnumber, the token circulation number of the received token frameincremented by one.
 13. The communication method according to claim 11,wherein in the first transmission right determining step, when the tokencirculation number in the token frame is equal to or larger than thetransmission permitted station value, the communication apparatusdetermines that the own station has the transmission right, in thesecond data frame transmitting step, when the second transmission rightacquisition determination information is larger than “0”, thecommunication apparatus determines that frames can be transmitted, inthe second data frame transmitting step, the communication apparatusattaches, to data that is in a transmission waiting state at the time ofthe token frame reception and could not be transmitted, last-timeun-transmitted data identification information for identifying the datathat could not be transmitted within a range of the second transmissionright acquisition determination information, in the second token frametransmitting step, when the second transmission right acquisitiondetermination information is “0” in the transmission of the data frame,the communication apparatus sets a value of the token circulation numberat the time of the token frame reception in number of transmittableframes “0” setting station information indicating the communication nodein the plurality of communication nodes that has set the secondtransmission right acquisition determination information to “0”, in thetransmission right determining step, when the transmission rightacquiring apparatus information in the received token frame indicatesthe own apparatus, the second transmission right acquisitiondetermination information is “0”, and the number of transmittable frames“0” setting station information has been set, the communicationmanagement apparatus determines that the transmission right has not beenobtained, and in the first token frame transmitting step, thecommunication management apparatus further sets the number oftransmittable frames “0” setting station information in the transmissionpermitted station value of the token frame.
 14. The communication methodaccording to claim 13, wherein, in the second data frame transmittingstep, when the communication apparatus has data attached with thelast-time un-transmitted data identification information, thecommunication apparatus transmits only the data attached with thelast-time un-transmitted data identification information at the time oftransmission right acquisition.
 15. The communication method accordingto claim 11, wherein, in the second data frame transmitting step, whenthe token circulation number is equal to or larger than the transmissionpermitting station value and the second transmission right acquisitiondetermination information is “0”, the communication apparatus attaches,to data that is in a transmission waiting state at the time of the tokenframe reception and could not be transmitted, last-time un-transmitteddata identification information for identifying the data that could notbe transmitted.
 16. The communication method according to claim 15,wherein, in the second data frame transmitting step, when thecommunication apparatus has data attached with the last-timeun-transmitted data identification information, the communicationapparatus transmits only the data attached with the last-timeun-transmitted data identification information at the time oftransmission right acquisition.
 17. The communication method accordingto claim 11, wherein, in the first token frame transmitting step, thecommunication management apparatus sets, as the second transmissionright acquisition determination information, a number of frames that canbe received at a point when the token frame in which the transmissionright acquiring apparatus information indicates the own apparatus isreceived.
 18. The communication method according to claim 11, wherein inthe transmission right determining step, when the communicationmanagement apparatus receives the token frame in which the secondtransmission right acquisition determination information is “0”, thecommunication management apparatus stores contents of the number oftransmittable frames “0” setting station information, and in the tokenframe transmitting step, when the second transmission right acquiringapparatus information is “0”, the communication management apparatussets, in the transmission permitted station value, changed destinationinformation during priority processing that is the sequence number ofthe communication apparatus to which the transmission right ispreferentially given.
 19. The communication method according to claim18, wherein when the second transmission right acquisition determinationinformation is “0”, the changed destination information during priorityprocessing includes a start transmission permitted station value and anend transmission permitted station value indicating the sequence numberin a range of the communication apparatus to which the transmissionright is preferentially given, and in the first transmitting rightdetermining step, when the token circulation number is present within arange indicated by the start transmission permitted station value andthe end transmission permitted station value, the communicationapparatus determines that the own station has the transmission right.20. The communication method according to claim 18, wherein, in thefirst token frame transmitting step, when contents of the number oftransmittable frames “0” setting station information are stored at thetime of reception of the token frame in which the transmission rightacquiring apparatus information indicates the own apparatus, thecommunication management apparatus sets the stored number oftransmittable frames “0” setting stating information in the transmissionpermitted station value.